Sir, ’twas not
Her husband’s presence only, called that spot
Of joy into the Duchess’ cheek; perhaps
Fra Pandolf chanced to say, “Her mantle laps
Over my lady’s wrist too much,” or “Paint
Must never hope to reproduce the faint
Half-flush that dies along her throat.
These lines show that she was enchanted by the people around her and that the painter "Fra Pandolf" caught the joy of her life as shown in "that spot of joy" that is seen on the Duchess' cheek. The Duke, of course, does not approve of her showing such "joy" in everything around her, because he believes that she should save all such emotion for him only.